I don't know if this is anything to be concerned about or not

Just finished a song off and decided to look at the "mastered" wave

Zoomed out, it certainly looks a bit squared off:









But zoomed in, it doesn't look quite so bad:








So, am I overcooking it? There's no audible distortion. Unsure of what to do - if anything!

Bristol it is nothing to be concerned about. You have not done a lot of mastering obviously. That would not be as loud as a commercial mastered track even. I have just finished mastering an album and it is all pretty squared off as you say but it sounds great and there is not distortion present anywhere. That is what a great limiter like the PSP Xenon can do. 

Don't sweat it. If you are doing work on a pro level for a client and they will want it as loud as commercial releases then you just have to resign yourself to the fact it ends up looking pretty square. I agree it would be nice to stop short of that last process but that is just not possible these days unfortunately

Ears and volume matched A/B ing is all you need to decide if it is too compressed. Forget the visuals.

I have noticed before that SONAR tends to make things look a lot more compressed than they look compared to some other programs. I have done wav editing using Goldwave in the past and I thought in SONAR it looked hyper hyper hyper compressed. Opened it up in Goldwave and it looked like I hadn't even put on a compressor!
Visuals are a representation. Let your ears be the final judge (volume matched, of course )

What you have there Jonesey, is the equivalent of walking into a barber shop and telling them to take a little off the top.  Then when you get home, the wife looks at you and says...I thought you were going to get a haircut today. 

All you have is very mild compression that has slightly trimmed the very peaks of the highest peaks.... nothing to worry about.  You could easily turn up the levels a bit and trim the tops some more and still not notice much of a difference..... but..... I would be very satisfied and happy with what you have right there......

I would not touch a thing.... based solely on the look of the wave of course.